2 answers

I generally try to avoid any of those scripts as these scripts needs JS to be activated and they slow the IE6 down a lot, if you have a couple of PNGs on your page. I always use GIFs within a CSS file for IE6 and lower which are not as nice as PNGs but they always work out. — Kau-Boy almost 9 years ago

When it's just a couple of PNGs, you're right, it can be better to use GIFs. It depends on whether your designer is happy for the images to not be as anti-aliased. Gifs do not always work however.
IE6 already runs very slowly indeed compared to most modern browsers anyway, an efficient script should not make too much difference.
There are now statistically less people using IE6 than there are people with JS turned off (according to today's mashable post) — danwellman almost 9 years ago